Nano structure science and technology is becoming a field of technology important to the economy. Nano particle production and its application is becoming a key aspect to old and emerging technology as more and more development to nano powder is occurring. Nano particles are a technology enabler having application in a vast array of industrial, domestic and commercial situations. For example nano particles are able to be utilised as a coating material to improve the wear resistance, the transparency and ultraviolet absorption as a coating to material. Nano particles can be used in the semiconductor industry to improve the polishing process of semiconductor components to achieve a high quality output. Nano particles are also utilised to increase the variety of gases that can be detected by a gas sensor as a result of increasing sensitivity. Nano particles can also be used in paint, cosmetics, and tooling industry. Nano particles have also found a place in the medical field and are utilised to enable drug delivery.
Nano particles have been classified as particles of a size less than 100 nm. It is at a particle size below 100 nm that such particles have displayed novel chemical and physical properties which in particle sizes above 100 nm are not (or are not as prominently) displayed by the material of the same or similar chemical makeup.
As a result of the small particle size a nano powder consisting of nano particles does not need to be heated to a temperature as high as larger particle size powder in order for it to be reactive for example for the coating of other products. There is accordingly an energy saving that can be provided by the use of nano powder thereby lowering the costs of the application of such materials to other materials. This is however only an extremely small part of the advantages due to its nano-size when used for spraying. The biggest advantages come from its very large relative surface area so that the material properties are dominated by surface properties instead of normal/classical bulk property.
The nano particle production industry currently utilises two distinct processes or a hybrid of such distinct processes. The first process is commonly known as the physical/dry process. The physical/dry process involves the subjecting of a precursor feed material or materials to an energy source.
The physical/dry process includes heat and mechanical energy as an energy source. The heat source is used to evaporate the precursor material or materials (referred to as the primary elements or compounds) and then reacts with secondary elements or compounds, if necessary to form the desired output compound. The heat source may include plasma, laser, flame, hot-box or oven, etc. The primary purpose of the heat source is to evaporate and dissociate the feed stock and provide necessary conditions for the subsequence reactions to carry out the synthesizing processes. The various heat sources have certain properties that may not be available by others. For example, laser provide clean environment but the cost of operation is high. Flame or a hot box provides an inexpensive heat source but is limited by its achievable temperature and cleanliness. A plasma heat source will provide a relatively inexpensive yet clean heat source.
Mechanical processing to create nano particles are also known and such can include a ball mill or grinding device as for example referenced to in U.S. Pat. No. 3,937,405. In mechanical processing the impact of the particles with like particles results in a reduction in particle size. Milling elevates the energy of the particles which both breaks down the particle size and allows for solid chemistry energisation for conversion of a compound to occur. Likewise U.S. Pat. No. 3,602,439 describes the use of a supersonic jet mill for producing extra fine and uniform powder from a precursor material by pulverising the material by mutual collision and friction of the material in a supersonic jet stream.
The second process technique for the creation of nano particles is a chemical/wet process which utilises the careful control of an environment to cause a reaction at the ionic level. Chemical processes known to those in the industry include colloid chemistry, sol-gel processes, precipitation processes such as hydrolysis or hydrothermal processes and micro emulsion.
Existing nano particle production devices which use a plasma source to evaporate material, pass the heated reacted gases and particle product through a convergent/divergent nozzle. The primary purpose of such a nozzle is to encourage the cooling of the particles that have been generated by their being subjected to the plasma energy source. The divergent part of the nozzle results in a decrease in pressure of the flow stream with a resultant reduction in stream temperature. Such cooling is known as adiabatic cooling. U.S. Pat. No. 4,484,943 describes such a nozzle.
Plasma energy is also known to be useful in production of reactive powder. For example particle spraying apparatus utilise a plasma source for generating the energy to create a high temperature sufficient to react the precursor material for the generation of small particles. They generally use a plasma screen which comprises of an arrangement for striking an electric arc between a pair of electrodes. A gas under pressure is passed between the pair of electrodes and through the arc to be passed through a nozzle. In a plasma arc torch of a non-transfer kind (unlike a transfer kind often utilised for cutting and welding) an arc is struck between a rear electrode (commonly a cathode) and a forward electrode that forms the exit nozzle of the plasma stream.
The common form of such non transfer torch, is a DC-plasma which is mainly used and has application for thermal spraying coating. The ‘exit nozzle’ serves the purpose to increase the velocity of the flow to achieve a good coating. U.S. Pat. No. 5,901,551 describes a torch where a jet nozzle has provided at its inlet a plasma generating electrode. The exit nozzle serves the purpose of accelerating the flow to high speeds such as to supersonic speeds.
U.S. Pat. No. 5,111,656, U.S. Pat. No. 5,640,843 and U.S. Pat. No. 5,901,551 disclose an arc jet nozzle through which the flow of fluid is a vortex flow. It has been identified in for example U.S. Pat. No. 5,901,551 that the purpose of the vortex is to enhance efficiency of the nozzle. The nozzle is used for the provision of thrust to the material stream passing therethrough. The nozzle described in U.S. Pat. No. 4,911,805 is mainly to accelerate the fine particle to high speed and strike onto the second material (target or substrate) and create coating. Commonly such nozzles are used in spraying technology and are used to control the flow stream and the velocity of the stream. Nozzles of spraying technology are not used for the cooling down of the particles flowing therethrough thereby inhibiting their growth and their inelastic collision capabilities such as required for sintering and do not endeavour to slow down or stop the flow and reaction of particles. This is not the objective of nano particle production.
Furthermore such nozzles are provided at the source of the plasma arc and do not include an intermediate reaction chamber between the plasma arc and the nozzle where the plasma subjected precursor feed material is able to react. Usually the vortex is provided for the purposes of enhancing the mixing of the gases, to create greater path length and also more uniform discharge to increase electrode lifetime. Increased lifetime results from the movement of the arc root (being a point discharge) by the vortex flow. If the arc root location does not change, the electrodes are easily pitted and hence shortened in life. The vortex through the arc jet nozzle will ensure the arc root moves from place to place and also stabilises the arc column. Hence the vortex is not used in cooling of the flow but to ensure the arc roots move from place to place and also stabilized the arc column.
Plasma torches come in the form of a DC/AC plasma torch which is a high power and high temperature device useful for the purposes of ensuring that high melting temperature materials can be subjected to sufficient energy to ensure that the precursor stock feed (often of a particulate or powder size) can be sufficiently softened/moltened and/or vaporised and accelerated to high velocities by the plasma device. Other forms of plasma torches include the RF plasma torch which is a lower energy density unit. RF (radio frequency) plasma provides cleaner but lower energy density plasma when compare to DC (direct current) plasma.
Current nano powder devices generally operate at supersonic speeds through the expansion nozzle. Operating at supersonic speeds provides for good quenching characteristics. High quenching characteristics of a nozzle is important in making small sized particles. There is a direct relationship between the degree of quenching and the resultant size of the particles. If less efficient quenching occurs through a nozzle the particle sizes will be larger i.e. the better the quenching the smaller the particle sizes. The degree of quenching may also be resultant in other characteristics such as particle shape or chemical makeup. As is well known in physical particle chemistry, particles colliding with each other in an elastic manner do not react to engage with each other however in an inelastic manner, particles colliding with each other will encourage particle growth. However for inelastic growth, particles need to have reached a certain kinetic energy. There is a minimum threshold energy level that is required to be achieved before particle growth will occur. Such minimum threshold energy level varies for different materials. As a result many nano powder production devices currently available are designed and set up for the production of one particular type of powder. Reaction chambers and nozzle shapes and speeds therethrough are setup particular for the production of specific nano particles. The degree of cooling also has a potential to change the resultant morphology of a particle as quenching rate will affect the material crystal growth. Very fast quenching will result in amorphous, non-crystal structure, material. However a problem exists with the volume of material that can be put through a convergent/divergent nozzle at supersonic speeds. This problem relates to the choking of the flow at supersonic speeds. Choking is a common phenomenon of supersonic flow nozzles. Whilst supersonic flow nozzles present great quenching characteristics, the total heat transfer is limited as a result of a reduced flow rate that can be put through the supersonic nozzle because of the choking phenomenon.
In order to achieve supersonic flow speeds, based on 1-dimensional flow with adiabatic expansion:
M2=(u/a)2={(P0/P)[(γ−1)/γ]−1}[2/(γ−1)] where u is the velocity of the fluid; a is the local acoustic velocity at the outlet; P0 is the upstream chamber pressure and P is the downstream chamber pressure; and γ is the ratio of specific heats of the fluid; and M (mach number) exceeds 1 (supersonic) when ratio P0/P is greater the critical ratio of pressure (>2 for ideal gas where γ=1.67). Similarly, the throat area is also governed by
A/A*=(1/M){[2/(γ+1)][1+M2(γ−1)/2])}[(γ+1)/2(γ−1)] where A and A* are the of diverging outlet and throat cross-section area.
In order to achieve high quench rate, the A/A* should be high and hence the throat cross-section is small that leads to constrain for the higher flow-rate.
U.S. Pat. No. 5,749,937 describes the use of a nozzle as a quencher which moves the material within the stream outside the “reaction threshold region” or sometimes called the “threshold thermal energy” for the purposes of titanium powder production from TiCl4. U.S. Pat. No. 5,749,937 however does not make reference to nano sized particle. U.S. Pat. No. 5,749,937 furthermore describes the additional gas injection as part of a reactant gas mixture for the purposes of completing the reaction but such occurs after the throat and downstream of the nozzle. Such additional gas injection is provided to avoid the back reaction by cooling the flow stream.
A further technique involves in the creation of nano particles is the forming of a powder both in the form of an alloy or in the form of a coating, also known as packaging. U.S. Pat. No. 4,687,611, U.S. Pat. No. 4,533,383, U.S. Pat. No. 4,484,943 and U.S. Pat. No. 5,093,148 describe the formation of such. U.S. Pat. No. 4,533,393 describes an alloying application of nano powder technology. U.S. Pat. No. 4,533,383 has the reaction of the two materials to be alloyed occurring downstream of the divergent/convergent nozzles. Synthesis occurs at the collision zone of the two flow streams after the nozzles. U.S. Pat. No. 4,484,943 is absent of reference to the use of a plasma jet for reacting with raw material. Furthermore only one heat source to heat the propellant gas, in this case nitrogen, is provided.
Packaging of small particles is described in U.S. Pat. No. 4,617,055. Two in series reaction chambers are described but again there does not appear to be a disclosure of two separate heat sources of a plasma jet kind. U.S. Pat. No. 4,617,055 describes a hotbox type heater for vapourising materials and this has the inherent disadvantages of contamination and temperature control hence affecting the output material. Commonly hotbox heaters are difficult to scale up as their heating elements are the limiting factor. Such are hence commonly used for batch production only. Furthermore, no ionisation occurs which is a phenomenon of plasma heating. As such the choice of raw materials is also more limited for hotbox type power producing machines.
In the two method as shown in U.S. Pat. No. 4,617,055, the second chamber is used mainly for controlling the chamber pressure ratio to achieve expansion cooling and control condensation temperature. A further method described in U.S. Pat. No. 4,617,055 is a series operation which has a lot of limitations such as the controlling of the second material evaporation and reaction with the first material, due to the hot box heating method. Another limitation seems to be that the operation of the first chamber is at 0.05 atm. The second chamber nozzle is fed by the first nozzle and the second furnace. Gas fed into the second chamber pressure has to be lower than the first chamber pressure in order to achieve the desired cooling rate. As the first chamber is running at 0.05 atm the second chamber pressure will have to be lower and hence limit the production rate and flexibility of operation. The reason the first chamber operates in such low pressure is due to the evaporation process for the hot box.
As a consequence the prior art known to the inventor has significant limitations in the types of materials that can be processed and produced as well as flexibility in adapting a particular device for the production of different sized, characteristic and/or compositions of materials. Known nano particle production devices are set up substantially permanently for the production of a particular nano particle which thereby limits the application that a particular device has and can result in a costly exercise in reconfiguring a machine for the production of different nano particles.
Accordingly it is an object of the present invention to provide a fine particle powder reactor and/or related process which has flexibility to allow for production of a number of alternative particles of varying composition or to at least provide the public with a useful choice.
It is also an object of the present invention to provide a quenching nozzle with improved efficiency over the prior art nozzles or at least provide the public with a useful choice.